The present invention relates to the art of making aluminum base alloy extruded products and is particularly concerned with extruded products which exhibit high strength and high electrical conductivity properties.
The extrusion process involves forcing the aluminum base alloy stock through a die opening having a predetermined configuration in order to form a shape having an indefinite length and a substantially constant cross-sectional area. This die extrusion process involves pre-heating the aluminum base alloy stock and placing the preheated stock in a cylinder which itself is usually heated. The cylinder has a suitable die placed at one end thereof and a reciprocating ram or piston arrangement having approximately the same cross-sectional dimensions as the bore of the cylinder. This ram is pressed against the heated alloy stock in order to compress the stock and thus force the alloy to flow through the die opening. Pressure exerted by the ram on the alloy stock during the compression operation raises the internal temperature of the stock as a result of internal friction within the stock body.
Properties of shaped aluminum base alloys formed from extrusion processes are generally determined by the temperature of the alloy before extrusion and the speed of extrusion. A balance between high alloy temperature combined with low extrusion speed on the one hand, and low alloy temperature combined with high extrusion speed on the other hand, should be struck for the most economical extrusion operation and consistent properties of the extruded product.
An important limiting factor in the extrusion of an aluminum base alloy is the extrusion speed. Too high a rate of speed causes what is known as chatter cracking in the extruded alloy material. This cracking phenomenon consists of surface defects in the extruded material which form a pattern of fine transverse cracks resulting from longitudinal tensile stresses which are quite high compared with the strength of the alloy at the working temperature. These fine cracks may not affect the overall strength of the extruded product but they do detract from the surface appearance, finishing ability, dimensional accuracy and mechanical integrity of the extruded product. It is well known that the chatter cracking phenomenon occurs at lower extrusion speeds as the extrusion temperature is raised. High strength aluminum base alloys must be extruded more slowly and at lower temperatures than regular aluminum or aluminum alloy materials in order to avoid the chatter cracking. This suggests that there is a relationship between flow stress and cracking tendency due to rises in extrusion surface temperature caused by adiabatic heating.